I just did a lot more reading. The pool guys say the brown algae is actually a form of mustard algae. They also say its chlorine 'resistant' and very difficult to kill, but thats still what they use to kill it.
They dont like algaecides because they add copper to the pool water - which is a no no in pools.
They recommend at least two to three heavy duty shock treatments in the 20PPM or higher range. They also recommend repeated brushing and vacuuming of the pool and scrubbing anything you can reach.
I dont have any algaecide at the moment, so I just dunked the entire root ball (its small) in some 60PPM pool shock for about 30 seconds. I also upped the PPM in the rez to 10 PPM. It will be diluted by the time it reaches the roots, so that should be safe - maybe.
I'll see how it looks tomorrow and maybe do another dunk - if the plant hasnt died yet If this doesnt work, then I will order some algaecide and try that.